Does the spread of hepatitis B virus genotype A increase the risk of intrafamilial transmission in Japan?

Recently, the prevalence of genotype A in patients with acute hepatitis B virus (HBV) infection has markedly increased in Japan. We encountered a 1-year-old infant who was infected with HBV genotype A and became an HBV carrier. His grandfather was identified as an HBV carrier; however, the grandfather was not aware of chronic HBV infection. This was a case of intrafamilial transmission. In addition, the child’s father developed acute hepatitis B within 1 month of the infant’s diagnosis. Molecular analysis revealed that the HBV isolates from the grandfather, the infant, and the father had identical sequences that belonged to genotypes A2/Ae. Compared with other HBV genotypes, genotype A has a significant association with chronic outcome. Therefore, prolonging hepatitis can increase the risk of transmitting the virus without realizing. The at-risk strategy of hepatitis B vaccination, which has been adopted in Japan, cannot prevent such intrafamilial transmission. Universal vaccination in childhood is only one way to prevent young children from unexpected HBV infection.